1. Field of the Invention
The present invention relates to an image forming system of electrophotographic type such as a copying machine, a laser beam printer (LBP) and the like, and more particularly, it relates to an image forming system having a plurality of image bearing members and capable of forming a multi-color image.
2. Related Background Art
Image forming systems wherein a multi-color image is formed by using a plurality of electrophotographic photosensitive drums (image bearing members) are already known. In such image forming systems, a number of process units each including a charger, an optical scanning device, a developing device, a cleaning device and the like which are disposed around a photosensitive drum are prepared in correspondence to the number of toner colors (for example, four sets when yellow toner, magenta toner, cyan toner and black toner are used), and a transfer belt (transfer sheet feeding and image transferring means) holding and feeding a transfer sheet is disposed below the process units so that toner images formed by the respective process units are successively transferred onto the transfer sheet moving along with the transfer belt in a superimposed fashion, thereby forming a multi-color image.
In such image forming systems, an image forming system wherein test density patterns (test patches) are transferred onto the transfer belt via the process units and the toner density of each test patch is detected so as to control the toner density in each developing device (particularly, when two-component toner comprising toner and carrier is used), thereby providing the proper image is known (as disclosed in the Japanese Patent Laid-Open Appln. No. 63-147177). Further, an image forming system wherein the test patches from the process units are superimposed on the transfer belt and the change in the magnitude of one dot in the image is measured on the basis of an amount of the positional discrepancy of each test pattern and the process conditions such as exposure time, output of image light, developing bias and the like are changed on the basis of the measured data to properly correct the magnitude of one dot, thereby providing the proper image is also known (as disclosed in the Japanese Patent Laid-Open Appln. No. 63-280275). Furthermore, an image forming system wherein the proper image can be formed even when the bright potential as the maximum reference density is changed due to the environmental variation, by estimating the bright potential from the test patterns having different potentials by changing the exposure amounts applied to the photosensitive drums is also known (as disclosed in the Japanese Patent Laid-Open Appln. No. 1-261668).
However, in the above-mentioned image forming systems capable of forming the multi-color image, although the toner density in each developing device, the magnitude of one dot in the image or the bright potential of each photosensitive drum varied in accordance with the environmental condition could be controlled by utilizing the test patches, the variation of the delicate color tone in the multi-color image due to the deterioration of the photosensitive drums, deterioration of toner and/or error in the control system could not be controlled. Accordingly, particularly, in the multi-color image including picture(s), the color balance including gray balance (density balance) was unstable.
The present invention intends to eliminate the above-mentioned conventional drawback, and an object of the present invention is to provide an image forming system which can obtain the proper image by controlling even the variation in the delicate color tone, when a multi-color image is formed on a transfer sheet carried by a transfer sheet feeding and image transferring means by using a plurality of image bearing members.
In order to achieve the above object, the present invention provides an image forming system having a plurality of color image forming stations, comprising a plurality of electrophotographic photosensitive members around which latent image forming means and color developing means are arranged, transfer sheet feeding means moved along an endless path and passed through transfer positions of the photosensitive members, means for forming pattern-shaped toner images on the photosensitive members, means for transferring the pattern-shaped toner images formed on the photosensitive members on the transfer sheet feeding means or on a transfer sheet fed by the transfer sheet feeding means, pattern-shaped toner image detecting means disposed in confronting relation to the transfer sheet feeding means and adapted to detect states of the toner images transferred to the transfer sheet feeding means or the transfer sheet fed by the transfer sheet feeding means, and image forming means for controlling an image forming condition on the basis of a detection result from the pattern-shaped toner image detecting means.